The invention relates to a control circuit for calibrating a delay line for a television receiver, which consists of series-connected all-pass sections of at least 2nd-order,the inductances of which are replaced by gyrators to which capacitances are connected.
Such a delay line containing 2nd-order seriesconnected all-pass sections is known from Valvo publication "Technical Information 840228" (TDA 4560). In all-pass sections, only the phase response of the transfer function depends on the frequency while the amplitude response is independent of frequency. For unmodulated signals, an all-pass section produces a delay which represents the phase delay. Since inductances are required for such all-pass sections which do not contain any active components, and these inductances cannot be integrated on a semiconductor substrate, the inductance has been replaced by a gyrator to which a capacitance is connected. This makes it possible to implement an all-pass section with the aid of transistors, capacitances and resistances on an integrated circuit.
The capacitances used are integrated as depletion layer capacitances. Since the vaalue of a depletion layer capacitance depends on the bias voltage applied to it, as is known, it is possible, on the one hand, to change the delay time of an all-pass section within a limited range by means of the bias voltage and, on the other hand, to use the voltage-dependence of the capacitances for automatically compensating for unwanted influences. In theknown circuit arrangement, a bias voltage can be derived by comparing an internal resistance with an external reference resistance which has a particular resistance value, which bias voltage is used for controlling the values of the depletion layer capacitances of the all-pass sections in such a manner that the delay time becomes independant of the doping and the geometric structural dimensions of the integrated circuit and of the temperature and of the supply voltage. However, it has been found in the production of such delay lines that variations occur which are so large that compensation of the unwanted influences is not completely achieved with the external reference resistance.